In the casting of aluminum ingots, the molten aluminum metal is maintained in a molten state in a holding furnace. Over time, a surface residue, or white dross, accumulates and must be skimmed from the surface of the molten metal. The white dross contains valuable materials which are recoverable by recycling processes. Typically, the white dross skimmings from a casthouse furnace may contain from about 30 to about 70% by weight aluminum, from about 20 to about 40% by weight oxides, from about 0 to about 6% by weight salts and from about 0 to about 4% by weight aluminum nitride. The oxides portion of white dross may contain from about 20 to about 40% Al.sub.2 O.sub.3, from about 15 to about 30% Al(OH).sub.3, from about 0 to about 3% by weight MgO, and from about 15 to about 30% by weight Al.sub.2 MgO.sub.4. The salts component is a mixture of NaCl and KCl.
This dross is treated with a salt flux which is a eutectic composition of KCl and NaCl, typically about 50 to 70% potassium chloride and about 30 to 50% sodium chloride, with an optional amount of up to about 5% by weight Na.sub.3 AlF.sub.6, in a rotary barrel furnace. Thus, for example, about 24,000 pounds of dross and about 13,200 pounds of salt flux are treated to remove the aluminum therefrom, yielding about 31,200 pounds of spent salt flux per cycle.
Traditionally, spent salt flux is permitted to solidify and sent to landfill for disposal. Thus, while spent salt flux contains valuable materials that could be recovered, it is currently the practice to pay to remove spent salt flux from its production site. It is also known to send spent salt flux to commercial salt recovery plants for recovery by milling, leaching, filtering and the like, but such recovery is very expensive, both in capital and processing costs.
It would make economic sense, therefore, to provide a low cost and low capital investment system to recover the valuable material components of spent salt flux, which would not only eliminate the cost of transporting and storing the material at a landfill or of expensive recovery at a traditional commercial salt recovery plant but at the same time result in a net cost savings by recovering valuable components of the spent salt flux for reuse.